fix: drain media before BYE on local hangup to prevent voicemail clipping

[rkfshakti]

Fixes livekit/livekit#4737

Problem

When an agent ends a call right after wait_for_playout() returns — EndCall RPC, end_call tool, or removing the SIP participant — the last word of the utterance is clipped on the callee's end (voicemail scenario). The audio is already past the agent: it is buffered in the room mixer (5-frame ~100ms input buffer), the codec encoder, and in-flight RTP. wait_for_playout() only tracks the local queue, so it cannot see this tail.

The SIP bridge then sends BYE and closes media immediately, so whatever is still buffered never clears the wire. A trailing pause masks the bug because it gives the tail time to flush; an abrupt end-of-speech hangup clips the last word. The reporter's workaround (2s pre-hangup sleep) confirms the mechanism.

Fix

Add a configurable drain window hangup_drain_time (default 500ms): on a locally-initiated clean hangup, keep feeding media to the peer for the window before sending BYE. The window is bounded (default 500ms) and only applies to:

• EndCall RPC (reason rpc)
• ctx cancel / agent end-call (reason hangup)
• participant removed from room (reason removed)
• cancelled invite (inbound, reason cancelled)

Remote BYE (the peer hung up), errors, timeouts, media failures, and pre-connect failures are not delayed. Set hangup_drain_time: -1 to disable.

Test

TestOutboundHangupDrainsMediaBeforeBYE: on an established outbound call, an EndCall RPC must not emit BYE before the drain window has elapsed. Fails on the old code (BYE sent before the media drain window elapsed), passes with the fix. All media-port and outbound tests pass; the pre-existing auth-test failures on clean main are unrelated (they fail identically without this change).

[rkfshakti]

Addressed the Devin review finding (cancelled inbound call delayed): drainOnHangup now gates on the media actually being bridged (c.started broken) and "cancelled" is dropped from the local-hangup set — a remote CANCEL before 200 OK is not a local hangup and gets no drain. Also constrained the media-port test range to 10000-20000 so CI can't land on a privileged port (<1024, seen on TestSetOfferReportsUnknownProvider: 'bind: permission denied').

[codecov]

Codecov Report

❌ Patch coverage is 70.00000% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 67.22%. Comparing base (0460b40) to head (a871b74).
⚠️ Report is 354 commits behind head on main.

Files with missing lines	Patch %	Lines
pkg/sip/inbound.go	66.66%	4 Missing and 1 partial ⚠️
pkg/config/config.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #811      +/-   ##
==========================================
+ Coverage   65.25%   67.22%   +1.97%     
==========================================
  Files          51       42       -9     
  Lines        6588     8226    +1638     
==========================================
+ Hits         4299     5530    +1231     
- Misses       1915     2205     +290     
- Partials      374      491     +117     

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